Switched-mode power supplies (SMPS) are commonly used and increasingly replacing “classical” power supplies composed of a transformer and a linear voltage regulator. SMPS use switching power converters to convert one voltage (e.g. a DC voltage provided by a battery) into another voltage, which may be used as supply voltage for an electric device or an electronic circuit. For example, switching power converters are widely used to convert a comparably high battery voltage of, for example, 12 volts to a lower voltage of, for example, 3.3 V. Such low voltages are needed to supply digital circuitry and signal processors used in automobiles or mobile devices such as mobile phones, portable computers, etc.
In many applications, high energy conversion efficiency is desired throughout a wide range of output currents. At high output currents, the predominant cause of losses is the on-state resistance of the semiconductor switches (power transistors) used in the switching power converter. The on-state resistance is basically inversely proportional to the active area of the power transistor. For a specific application, a minimum chip area can be calculated for a given desired on-state resistance or for a desired energy conversion efficiency.
Generally, larger transistors (with a larger active area) have lower on-state resistances and therefore allow higher energy conversion efficiency for high output currents. However, larger transistors entail higher intrinsic capacitances, which adversely affect the energy conversion efficiency. At low output currents, the predominant cause of losses is the charging and discharging of the intrinsic capacitances of the power transistors. As a consequence, the circuit designer faces a conflict of objectives, as a high energy conversion efficiency at high output currents (i.e. at full load) is detrimental for a high energy conversion efficiency at low output currents (i.e. at light load); and many circuits operate at low currents (stand-by, power saving mode, etc.) for most of the time.
For switching converters with a synchronous rectifier the dead time between the switch-off of the low side-switch and a subsequent switch-on of the high-side switch (and vice versa) is also relevant to power losses and thus modern driver circuits may control the operation of the switching power converter such that the mentioned dead time is (approximately) at a minimum. However, a minimum dead-time is needed to avoid cross-conduction.